Electronic musical instrument tone generator having vibrato effect



Feb. 8, 1966 M. GRASER. JR 3 234,485

ELECTRONIC MUSICAL INSTRUMENT TONE GENERATOR HAVING VIBRATO EFFECT Filed April 4, 1965 TUNNEL DIODE 0 l9 I8 I 3 j .2

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" FI/ U H J H TIME FlG.3c 24 24 TIMVE F|G.4 i 7 M 'fvvv W l6 8 4 a g g E 6 INVENTORI Q, MICHAEL GRASER JR., J

HIS ATTORNEY.

United States Patent 3,234,425 ELECTRONIC MUSKCAIL INSTRUMENT TONE GENERATUR HAVENG VIBRATQ EFFECT Michael Graser, In, Fayetteville, N.Y., assignor to General Electric Company, a corporation of New York Filed Apr. 4, 1963, Ser. No. 270,761 1 Claim. (Cl. 33l1tl'7) This invention relates to electronic musical instruments arranged to provide a vibrato effect.

With the exception of a pure tone, such as produced by a vibrating tuning fork, tones generally may be subdivided into a lowest frequency signal known as a fundamental, and a plurality of higher frequency signals known as overtones. Normally, the frequencies of the overtones are integral multiples, or harmonics, of the fundamental frequency.

As shown by H. von Helmholtz in 1859, the quality of any tone depends upon the number, the intensities, and the frequencies of all the overtones present with the fundamental. It is the quality of any tone which enables one with a trained musical ear to recognize the source as being one of a plurality of musical instruments, such as a violin, organ, flute, or clarinet. In order to electronically synthesize a tone having the quality characteristic of a particular musical instrument, an electronic signal must be provided having a frequency equal to the frequency of a desired fundamental and including a plurality of harmonies corresponding to the overtones which characterize the instrument. Successful synthesis has been achieved using combinations of pure tones, first by D. C. Miller, using a set of organ pipes, and more recently by Harvey Fletcher, with a group of electronic circuits that enable mixing in any proportions 100 pure tones spaced over the principal audio range.

The investigations of Helmholtz and others more recent tend to show that the relative phases of the component signals are immaterial when translated into psychological quantities bearing upon auditory sensations in the brain. Thus, the exact waveform synthesized is not important, per se, but is significant only as it relates to the harmonics contained in the signal generated. Therefore, as an alternative to combining a large number of pure tones to achieve a composite signal of the requisite quality, it is possible to use an electronic signal generator having an output which is other than a pure tone, and rich in the harmonics of any desired fundamental frequency. Thereafter, the harmonics may be selectively filtered to achieve the characteristic quality of any desired musical instrument. This method of tone generation has the advantage of'requiring only a single electronic generator to synthesize a desired tone.

From the theorem discovered by the French mathematician Joseph Fourier in the year 1807, it is known that a square-wave electrical signal can be expressed as the sum of a fundamental frequency plus all of the odd harmonics thereof. As the generated waveform tends away from a square-wave and toward a rectangular pulse waveform, the relative amplitude of the fundamental frequency decreases and even harmonics become noticeable.

The sound of a pipe organ presents a tone having all the odd harmonics, in the case of a closed pipe, and having all of the even and odd harmonics, in the case of an open pipe. Therefore, it is desirable to provide an electronic signal generator having an output signal which may be easily and precisely varied between square-waves and pulses, in order to synthesize the wide limits of sound produced by an organ. Sounds characteristic of other musical instruments may be synthesized by filtering the output signal. Such an electronic tone generator may be referred to generically as a rectangular-wave generator of variable pulse width.

It is frequently possible in musical instruments to produce a sound having a particular variable quantity, which may be described generally as a musical embellishment. One such variable effect commonly associated with musical instruments is vibrato, which may be described as a musical embellishment which depends primarily upon periodic variations of frequency which are often accompanied by variations in amplitude and waveform. The quantitative description of vibrato is usually in terms of the corresponding modulation of frequency, waveform and amplitude of the fundamental. In the interest of realistic synthesis of the sound of musical instruments, it is highly desirable to provide some means whereby an electronic musical instrument may also synthesize the musical embellishment of vibrato.

Accordingly, it is an object of this invention to provide an electronic rectangular-wave generator of variable pulse width which is capable of synthesizing the musical embellishment of vibrato.

It is another object of this invention to provide an electronic rectangular-wave generator wherein the frequency, waveform and amplitude of fundamental of the signal generated may be adjusted and periodically varied.

Briefly summarized, in one embodiment of this invention, a tunnel diode relaxation oscillator is utilized as a rectangular-wave generator. The oscillator circuit of the generator comprises a tunnel diode, an inductor, and a resistor. The sum of the resistance of the resistor plus the resistance of the inductor must be less than the negative resistance of the tunnel diode. Thereafter, when a direct current power supply of suitable voltage magnitude such that the DC. load line intersects the tunnel diode characteristic in the negative resistance region is connected to the oscillator circuit, a voltage of rectangular waveform is generated across the tunnel diode. By varying the source voltage the rectangular waveform is adjusted to provide a series of short pulses, a square-wave, or any intermediate configuration. A transformer secondary winding is disposed in series with the source in order to provide a slow modulation of the source voltage in response to a periodically varying voltage supplied to the transformer primary winding. The oscillator circuit is responsive to the modulated source voltage to provide corresponding periodic variations of the frequency, waveform and amplitude of fundamental of the signal appearing across the tunnel diode, thereby achieving a vibrato effect.

FIG. 1 is a schematic circuit diagram of the preferred embodiment of my invention.

FIG. 2 is the static characteristic curve of a tunnel diode,

FIGS. 3a, 3b, and 3c show some of the various voltage waveforms developed across the tunnel diode of FIG. 1, and

FIG. 4 is a schematic diagram of an alternative embodiment of my invention.

FIG. 1 illustrates a tone generator having a tunnel diode relaxation oscillator circuit 1. The oscillator circuit comprises a series loop network including a tunnel diode 2, an inductor 3 and a resistor 4. The output voltage is developed across tunnel diode 2 and is supplied to external circuitry through terminals 5 and 6.

In order to provide the necessary electrical energy for circuit operation, a power supply circuit is provided including a direct current voltage source 7 which, for example, may be a battery. Voltage source 7 supplies electrical energy to oscillator circuit 1 through variable resistor 8 and fixed resistor 9. In addition, the power supply circuit includes a serially disposed secondary winding 10 of transformer 11. The transformer is adapted to be connected to an external source through terminals 12 and 13 which are connected to opposite extremities of transformer primary winding 14. 1

In order to provide vibrato effect, modulation of the power supply voltage is achieved by applying a slowly varying signal to terminals 12 and 13 of transformer primary winding 14. A slowly varying voltage is thereby induced in transformer secondary winding it), whereby the effective power supply voltage, as represented by the algebraic summation of the voltage of source 7 and the voltage induced in secondary winding 10, is caused to vary above and below the voltage of source 7 in ac cordance withthe slowly varying voltage applied to terminals 12 and 13.

Variable resistor 8 normally has a relatively low resistance value and allows fine adjustment of the volt-age supplied to oscillator circuit 1. Resistor 9 limits the current flowing to oscillator circuit 1 and its resistance value depends upon the voltage of source 7 and the various para-meters of oscillator circuit 1. Resistor 9 also provides isolation of oscillator circuit 1 from other oscillators to which energy may be supplied simultaneously from the power supply through terminal which is connected to junction 16 of variable resistor 8 and resistor 9.

The theory of operation for tunnel diode relaxation oscillators, such as shown in circuit 1, is fully described in detail in the General Electric Tunnel Diode Manual, published in 1961 by the assignee of the present invention, commencing on page 50. In view of the teaching in the aforementioned publication, the basic operation of oscillator circuit 1 will be described only briefly herein.

Referring to the tunnel diode static characteristic curve of FIG. 2, current through the tunnel diode increases exponentially to peak point 17, when power is supplied to oscillator circuit 1. The time required for the tunnel diode current to reach the magnitude at peak point 17 is dependent upon the magnitude of power supply voltage, the resistance value of resistor 4 and the inductance value of inductor 3.

D.C. load lines 18, 19 and 20 each represent a particular combination of source voltage and load resistance. The slope of the load lines equals the negative reciprocal of the load resistance which is defined as the resistance of the inductor plus the parallel resistance of resistors 4 and 9. The intersections 21, 22 and 23 of the abscissa and of load lines 18, 19 and 20, respectively, occur at a voltage equal to the magnitude of source voltage selected for each line. The load resistance must have a value less than the negative resistance exhibited by tunnel diode 2 and each D.C. load line intersects the tunnel diode characteristic in the negative resistance region, as required for unstable biasing of the tunnel diode and circuit oscillation.

When the tunnel diode current reaches the value at peak point 17, there is a rapid switch to the high voltage "state shown at 24. This transition is so rapid that normally the voltage may be considered to jump instantaneously as indicated by the dashed line between point 17 and point 24. Thereafter the tunnel diode current decays exponentially to valley point 25. Following the decay to point 25, a rapid switch occurs to the low voltage state shown at 25, as indicated by the dashed line between point 25 and point 26. The tunnel diode current'once again exponentially increases toward peak point 17, and another cycle is thereby commenced like that previously described.

FIGS. 3a, 3b, and 3c show the voltage waveform appearing across tunnel diode 2 and supplied to terminals 5 and 6, for the various load lines 18, 19 and 20, respectively, of FIG. 2. Since load lines 18, 19 and 20 are of equal slope and therefore represent D.C. load lines of equal load resistance, the waveforms of FIGS. 3a, 3b and show the waveforms of voltage across tunnel dide 2 as a function of changes in the bias of tunnel diode 2 caused by variation of the supply voltage, such as indicated at points 21, 22 and 23.

Points 17, 24, 25 and 26,- of FIGS. 3a, 3b and 3c, each correspond to the voltage developed across the tunnel diode when the diode is operating at corresponding numbered points on its static characteristic curve, as shown in FIG. 2. In FIG. 3a, which corresponds to circuit operation using load line '18, the waveform is seen to closely approximate a square-wave. The essential characteristics of the square-wave are that the shape is generally rectangular, and that the time occupied at the low voltage state, as represented by the charging time between point 26 and point 17, is substantially equal to the time occupied at the higher voltage state, as represented by the discharge time between point 24 and point 25. As the supply voltage is decreased circuit operation occurs using D.C. load line 19, and the waveform of FIG. 3b is reached.

Comparing the waveforms of FIG. 3a and FIG. 3b, it will be noted that the charge time between point 26 and point 17 has been substantially increased relative to the-discharge time between point 24 and point 25. The waveform is still of generally rectangular configuration; however, it may no longer be characterized as a square-wave since the charge and discharge times are greatly different. It will be noted that the charge time between point 26 and point 17 is highly dependent upon the magnitude of supply voltage and increases as the supply voltage decreases. The discharge time between point 24 and point 25, on the other hand, is less sensitive to the magnitude of supply voltage.

The pronounced effect of decreasing supply voltage, and resulting change in the biasing of tunnel diode 2, is even more noticeable when comparing FIGS. 3a and and 30. FIG. 3c represents circuit operation when the D.C. load line is as shown at 20 in FIG. 2. The charge time from point 26 to point 17 in FIG. 3c may be seen to be many times that of FIG. 3a, whereas the discharge time from point 24 to point 25 is somewhat less changed.

The dependence of oscillator circuit 1 upon the magnitude of supply voltage may be summarized as follows. At some predetermined voltage, slightly less than the valley voltage of the tunnel diode, the output voltage waveform is approximately a square-wave. A decrease in supply voltage causes a decrease in frequency and a progressive departure of the output voltage waveform from the square-wave toward a narrow pulse waveform. Although not illustrated, there is a narrow range of supply voltage a-bove the predetermined voltage wherein the frequency increases and the discharge time'exceeds the charge time. The duty cycle of the waveform, which is proportional to the percentage of time occupied at the higher voltagestate, and the frequency of the waveform vary directly with changes in supply voltage.

Translating the above-described relationship into a practical tone generator for an electronic musical instrument, it is helpful to make reference to Fouriers theorem. Applying the theorem to a rectangular waveform, as explained and discussed in many texts concerned with waveform analysis, shows that the square-wave of FIG. 3a may be considered to comprise a direct current average potential, which may be neglected, a sine Wave of fundamental frequency equal to that of the square-wave, and all of the odd harmonics of the fundamental. The tone generated by a closed pipe in an organ has a similar frequency spectrum. Since the phase relationships in a tone comprising a plurality of harmonics are of no substantial consequence to the listener, the squarewave of FIG. 3a may be converted by an electric-acoustic transducer into sound which closely approximates that emanating from a closed organ pipe source. Therefore, whenever it is desired to electronically synthesize the sound of a closed organ pipe source, variable resistor S, of FIG. 1, is adjusted to provide circuit operation using load line 18 of FIG. 2. The resulting waveform is then that of FIG. 3a.

Apply-ing Fouriers theorem to the rectangular waveform of FIG. 3b, which corresponds to operation using load line 19, shows that the even harmonics of the fundamental frequency are of noticeable magnitude, although less substantial than the odd harmonics. Also, the relative amplitude of the fundamental frequency component has decreased in the waveform of FIG. 3b.

These effects are progressive, and the waveform of FIG. 3c may be shown to be rich in all of the harmonics of a fundamental sine wave having a frequency equal to the pulse recurrence rate. Also, the relative magnitude of the fundamental frequency is further decreased from its magnitude in either of FIGS. 3a or 3b. This is so even though the voltage excursion of the waveform has remained constant. When it is desired to electronically synthesize a musical instrument which is rich in all of the harmonics of a particular tone, such as an open organ pipe source, variable resistor 8 is adjusted to provide oscillator circuit operation using load line 20, of FIG. 2.

While the output voltage, frequency, waveform and amplitude of fundamental have been found not to be linearly proportional to source voltage over the entire range bounded by load line 18 and load line 20, a smooth and continuous transition has been found to occur between the Waveform of FIG. 3a, when using load line 18, and the waveform of FIG. 3c, when using load line 20. Therefore, when vibrato effect is to be provided, a slowly varying alternating current is supplied to terminals 12 and 13 of primary winding 14. The effective variation of power supply voltage above and below the value predetermined by setting variable resistor 8, and resulting change in the bias of tunnel diode 2, then introduces a periodic change in frequency of the output voltage across tunnel diode 2. The output voltage slowly varies in frequency, waveform and amplitude of the fundamental. Such a variation, when converted to sound by an electro-acoustic transducer, produces the requisite characteristics of that musical embellishment known as vibrato.

FIG. 4 shows an alternative embodiment of my invention wherein oscillator circuit 1 is provided with a series power supply connection, as opposed to the parallel power supply connection featured in FIG. 1. The oscillatory circuit functions substantially as described before; however, it will be noted that current limiting resistor 9 is no longer used. With the series power supply connection, all the current of oscillator circuit 1 passes through the power supply and therefore a large impedance, such as current-limiting resistor 9, would prevent circuit oscillation if included in the circuit. The circuit of FIG. 4 has the advantage of being more economical to manufacture than that of FIG. 1. However, when the power supply is used to serve a plurality of oscillator circuits decoupling techniques must be employed to prevent interaction. The circuit arrangement of FIG. 1, on the other hand, makes easy the necessary isolation of oscillation circuits, one from the other, by choosing a high enough source voltage such that resistor 9 may have several times the resistance value of resistor 4.

There has been shown and described herein a tone generator, for an electronic musical instrument, which is responsive to a small change in power supply voltage to provide an electronic output signal having the necessary characteristics for providing a synthesized musical signal covering a full range of sound. In addition, the tone generator is responsive to small periodic variations, or modulations, of the power supply voltage to provide a syn thesis of that musical embellishment known as vibrato. The power levels in the circuit are extremely low, allowing the use of low cost, compact elements. A one octave organ was constructed utilizing the circuit of FIG. 1 with the addition of seven other oscillator circuits. The entire electronic organ circuitry, including accompanying R-C filtering means and associated transistor amplifiers, was mounted in the keyboard, with the exception of an associated loud-speaker and battery power source which were mounted externally. A six c.p.s. signal of only .2 volt peak-to-peak across secondary winding 10 of transformer 11 was found to provide a satisfactory and adequate vibrato effect.

While the preferred embodiments of this invention have been disclosed, many modifications and variations of this invention are possible and will occur to those skilled in the art. Therefore, it is intended that the scope of the subject invention be defined solely by the following claim.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

An electronic musical instrument tone generator arranged to provide a vibrato effect comprising:

(a) a first series loop network including a tunnel diode, an inductor, and a load resistor, said load resistor having a resistance value less than the value of the negative resistance exhibited by said diode, said first loop network being adapted to cause a voltage of rectangular waveform of the type which can be used to produce musical tones to be generated across said tunnel diode when said tunnel diode is biased in an unstable manner;

(b) a second series loop network including said load resistor, a transformer secondary winding, a directcurrent voltage source, a variable resistance means for adjusting the bias of the tunnel diode, and a current-limiting resistor connected to said load resistor having a resistance value to provide an unstable biasing of said tunnel diode, the resistance value of said current-limiting resistor being several times greater than the resistance value of said load resistor;

(c) means for inducing a slowly varying voltage in said transformer secondary winding of a magnitude which provides a corresponding variation in the biasing of said tunnel diode, said tunnel diode being responsive to said variation in bias to vary the duty cycle and frequency of said voltage of rectangular waveform and thereby provide a vibrato effect.

References Cited by the Examiner UNITED STATES PATENTS 2,407,424 9/ 1946 Hollingsworth 332-30 2,902,655 9/1959 Jones et al 84-101 2,933,697 4/1960 Oncley 332-30 3,076,944 2/ 1963 Watters 331-107 3,134,949 5/1964 Tiemann 3323O OTHER REFERENCES Watters et al.: The Tunnel Diode Story, Electronics, July 1960, pp. 26-29.

General Electric Tunnel Diode Manual, March 20, 1961, pp. 50-52.

ROY LAKE, Primary Examiner.

S. H. GRIMM, Assistant Examiner. 

